Origen

Late Middle English (in the sense 'overflow'): from Old Frenchsouronder, from late Latinsuperundare, from super- 'over' + undare 'to flow' (from unda 'a wave'); later associated with round. Current senses of the noun date from the late 19th century.

‘Overflow’ was the early meaning of surround. It came via Old French, from late Latin superundare; formed from the elements super- ‘over’ and undare ‘to flow’. The meaning altered under the influence of ‘round’. Military use (‘enclose on all sides so as to cut off’) arose in the mid 17th century. See also water